Article last updated on: Jan 25, 2019

What are quantum dots?

Quantum dots, or QDs, are semiconductor nanoparticles or nanocrystals, usually in the range of 2-10 nanometers (10-50 atoms) in size. Their small size and high surface-to-volume ratio affects their optical and electronic properties and makes them different from larger particles made of the same materials. Quantum dots confine the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions. Quantum dots are also sometimes referred to as ‘artificial atoms’, a term that emphasizes that they are a single object with bound, discrete electronic states, similarly to naturally occurring atoms or molecules.

Quantum Dots - fluorescent image

Many types of quantum dot are fluorescent - they emit light of specific frequencies if electricity or light is applied to them. These frequencies can be tuned by changing the dots' size, shape and material, opening the door to diverse applications. Generally speaking, smaller dots appear blue while larger ones tend to be more red. Specific colors also vary depending on the exact composition of the QD.

Applications

Thanks to their highly tunable properties, QDs are attracting interest from various application developers and researchers. Among these potential applications are displays, transistors, solar cells, diode lasers, quantum computing, and medical imaging. Additionally, their small size enables QDs to be suspended in solution, which leads to possible uses in inkjet printing and spin-coating. These processing techniques may result in less-expensive and less time consuming methods of semiconductor fabrication.

Quantum dots are considered especially suitable for optical applications, thanks to their ability to emit diverse colors, coupled with their high efficiencies, longer lifetimes and high extinction coefficient. Their small size also means that electrons do not have to travel as far as with larger particles, thus electronic devices can operate faster. Examples of applications that take advantage of these electronic properties include transistors, solar cells, quantum computing, and more. QDs can greatly improve LED screens, offering them higher peak brightness, better colour accuracy, higher color saturation and more.



QDs are also very interesting for use in biomedical applications, since their small size allows them to travel in the body, thus making them suitable for applications like medical imaging, biosensors, etc.

What is graphene?

Graphene is a material made of carbon atoms that are bonded together in a repeating pattern of hexagons. Graphene is so thin that it is considered two dimensional. Graphene's flat honeycomb pattern gives it many extraordinary characteristics, such as being the strongest material in the world, as well as one of the lightest, most conductive and transparent. Graphene has endless potential applications, in almost every industry (like electronics, medicine, aviation and much more).

Graphene structure photo

The single layers of carbon atoms provide the basis for many other materials. Graphite, like the substance found in pencil lead, is formed by stacked graphene. Carbon nanotubes are made of rolled graphene and are used in many emerging applications from sports gear to biomedicine.

Graphene quantum dots

The term graphene quantum dots (GQDs) is usually used to describe miniscule fragments, limited in size, or domains, of single-layer to tens of layers of graphene. GQDs often possess properties like low toxicity, stable photoluminescence, chemical stability and pronounced quantum confinement effect, which make them attractive for biological, opto-electronics, energy and environmental applications.

The synthesis of graphene quantum structures, such as graphene quantum dots, has become a popular topic in recent years. While graphene usually does not have a bandgap - which is a problem for many applications - graphene quantum dots do contain a bandgap due to quantum confinement and edge effects, and that bandgap modifies graphene's carrier behaviors and can lead to versatile applications in optoelectronics. GQDs were also found to have four quantum states at a given energy level, unlike semiconductor quantum dots, which have only two. These additional quantum states, according to researchers, could make GQDs beneficial for quantum computing.

Additional properties of GQDs such as high transparency and high surface area have been proposed for energy and display applications. Because of the large surface area, electrodes using GQDs are applied for capacitors and batteries.

Various techniques have been developed to produce GQDs. Top-down methods include solution chemical, microwave, and ultrasonic methods. Bottom-up methods include hydrothermal and electrochemical methods.

Further reading

The latest graphene quantum dots news:

New strategy uses graphene quantum dots to boost catalysts

Rice University researchers, in collaboration with teams at Oak Ridge National Laboratory, University of Saskatchewan, King Abdullah University of Science and Technology and CAS, have used graphene quantum dots (GQDs) to assemble what they say may transform chemical catalysis by greatly increasing the number of transition-metal single atoms that can be placed into a carbon carrier.

General synthesis of single-atom catalysts with high metal loading using graphene quantum dots imageThe process uses functionalized graphene quantum dots to trap transition metals for higher metal loading single-atom catalysis. Illustration courtesy of the Wang Group (from Rice Uni website)

The technique uses graphene quantum dots, 3-5-nanometer particles of graphene, as anchoring supports. These facilitate high-density transition-metal single atoms with enough space between the atoms to avoid clumping.

Qurv Technologies develops graphene-enabled wide-spectrum image sensor technologies

Qurv Technologies, a Spain-based startup established in 2020 to develop wide-spectrum image sensors based on graphene and quantum-dot technologies, has developed a sensor that combines the unique electronic properties of graphene with suitable quantum nanoparticles as light sensitizers. The new device reportedly enables efficient detection of a broad range of wavelengths – from ultraviolet to infrared light – all concentrated into one simple device.

The production and transfer process of graphene leverages existing scalable Complementary Metal Oxide Semiconductor (CMOS) manufacturing processes. Furthermore, this graphene-based sensor could replace traditional costly alternatives based on indium gallium arsenide, paving the way to SWIR imagers up to 1000 times cheaper.

GS Alliance develops a graphene QD and silica composite to create efficient white LEDs

Japan-based material developer Green Science Alliance developed a new composite material made from a combination of graphene quantum dots and silica, which could be useful for creating white LEDs from blue LEDs (440-470 nm).

Green Science Alliance graphene QD + silica composite for white LED

The company says that this is the first adoption of such a material for LED applications. The company says the adoption of the QD and Silica offers superior performance to the currently-used phosphor as the QDs do not suffer from light scattering, and the white LED is more efficient. The material is also said to be inexpensive.

ZEN Graphene Solutions and NPNL jointly receive NSERC grant

Zen Graphene Solutions logo imageZEN Graphene Solutions, in partnership with Professor Mohammad Arjmand, has announced the award of a CAD$780,000 (around USD$609,600) Alliance Grant - part from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the rest from a combination of cash and in kind contributions from ZEN).

Alliance Grants are awarded through a competitive peer review process, and this proposal, titled “Synthesis of Graphene Nanomaterials and Development of Their Multifunctional Polymer Nanocomposites”, is ZEN’s highest single monetary grant award from NSERC to date and supports NSERC’s growing interest in nanomaterials.

Researchers find that graphene quantum dots are biodegradable by human enzymes

A study recently conducted by Graphene Flagship partners the University of Strasbourg and CNRS, France, in collaboration with Nanyang Technological University in Singapore, has shown that graphene quantum dots are biodegradable by two enzymes found in the human body.

Graphene quantum dots (GQDs) are tiny flakes usually smaller than five nanometres that have potential for many applications. GQDs are fluorescent, so they can absorb light and then emit it, often at a different wavelength. They are also so small that they can penetrate cells. Together, these properties pave the way to a wide array of applications in bioimaging, biosensing and new therapies - among other potential uses.